What exactly is the open phone service Asterisk and what does it do?

What exactly is the open phone service Asterisk and what does it do?

Hi, I was just curious to this Asterisk service I found. I kind of came upon it by accident through a chat forum where someone was mentioning it was a great service that didn’t have all of the hassles as regular phone company service. I’m not sure exactly what it is or what it does though…is it for home lines or more like a business? Is there a lot of programming and parts to buy?
I’d really appreciate the help if anyone knows the answers to this, thanks!